332 THE CHICK. 



(ii) The membrane-bones are less closely connected with the 

 cartilaginous skull than are the cartilage-bones, and can only be 

 grouped somewhat arbitrarily according to the primary divisions 

 of the cartilaginous skull. 



In connection with the cranium and the sense capsules are 

 formed the parietals, squamosals, frontals, laerymals, nasals, 

 vomer, lasitemporal, and parasphenoid. 



In connection with the upper jaw are formed the pterij- 

 goids, palatines, quadratojugals,jugals, maxillos, and premaxlllce. 



In connection with the lower jaw are formed on each side the 

 dentary, angulare, supra-angulare, and splenial. 



3. The Pectoral Girdle and Sternum. 



The sternum develops as two separate halves, apparently 

 formed by fusion of the ventral ends of the ribs, which meet 

 and unite in the median plane on the ninth or tenth day. Both 

 halves contribute to the formation of the keel, which is formed 

 by fusion of their adjacent edges. The keel is very small until 

 towards the close of development. The manubrium of the sternum 

 is formed rather later, and is apparently a secondary outgrowth. 



The sternum shows, in its development, evidence of having 

 been originally of greater length, and associated with a larger 

 number of ribs than in the adult fowl. In embryos of the 

 sixth day the two hindmost cervical ribs are attached to the 

 sternum ; on the seventh day they have lost their sternal attach- 

 ments, but are still greatly elongated. At the hinder end of the 

 thoracic series there is, during the sixth and seventh days, a 

 rudiment of an eighth rib, which disappears shortly afterwards. 



In the shoulder girdle, the scapula and coracoid are almost at 

 right angles to each other on the seventh day, the scapula being 

 long and blade-like, and the coracoid short and stout. At the 

 beginning of the sixth day the scapula and coracoid are con- 

 tinuous with each other, but before the end of the day they 

 separate. 



The clavicles are membrane-bones ; the median part of the 

 furcula has been compared to an interclavicle, but the embryo- 

 logical evidence does not support this view. 



4. The Fore-limb or Wing. 



The humerus, and the radius and ulna, present no points of 

 special importance in their development ; but the carpus and the 



